Method for electronically adjusting the selective oscillation frequency of a coriolis gyro

ABSTRACT

In a method for electronic tuning of the frequency of the read oscillation to the frequency of the stimulation oscillation in a Coriolis gyro ( 1 ′) according to the invention, the resonator ( 2 ) of the Coriolis gyro ( 1 ′) has a disturbance force applied to it such that the stimulation oscillation remains essentially uninfluenced, with the read oscillation being changed such that a read signal which represents the read oscillation contains a corresponding disturbance component. The frequency of the read oscillation is controlled such that the phase shift between the disturbance signal and the disturbance component which is contained in the read signal is a minimum.

The invention relates to a method for electronic tuning of the frequencyof the read oscillation to the frequency of the stimulation oscillationfor a Coriolis gyro.

Coriolis gyros, (which are also referred to as vibration gyros) arebeing used to an increasing extent for navigation purposes; they have amass system which is caused to oscillate. This oscillation is generallya superimposition of a large number of individual oscillations. Theseindividual oscillations of the mass system are initially independent ofone another and can each be regarded in an abstract form as“resonators”. At least two resonators are required for operation of avibration gyro: one of these resonators (first resonator) isartificially stimulated to oscillate, with these oscillations beingreferred to in the following text as a “stimulation oscillation”. Theother resonator (the second resonator) is stimulated to oscillate onlywhen the vibration gyro is moved/rotated. Specifically, Coriolis forcesoccur in this case which couple the first resonator to the secondresonator, draw energy from the stimulation oscillation of the firstresonator, and transfer this energy to the read oscillation of thesecond resonator. The oscillation of the second resonator is referred toin the following text as the “read oscillation”. In order to determinemovements (in particular rotations) of the Coriolis gyro, the readoscillation is tapped off and a corresponding read signal (for examplethe tapped-off read oscillation signal) is investigated to determinewhether any changes have occurred in the amplitude of the readoscillation which represent a measure for the rotation of the Coriolisgyro. Coriolis gyros may be in the form of both an open loop system anda closed loop system. In a closed loop system, the amplitude of the readoscillation is continuously reset to a fixed value—preferably zero—viarespective control loops.

In order to further illustrate the method of operation of a Coriolisgyro, one example of a closed loop version of a Coriolis gyro will bedescribed in the following text, with reference to FIG. 2.

A Coriolis gyro 1 such as this has a mass system 2 which can be causedto oscillate and which is also referred to in the following text as a“resonator”. This expression must be distinguished from the “abstract”resonators which have been mentioned above, which represent individualoscillations of the “real” resonator. As already mentioned, theresonator 2 may be regarded as a system composed of two “resonators” (afirst resonator 3 and a second resonator 4). Both the first and thesecond resonator 3, 4 are each coupled to a force transmitter (notshown) and to a tapping-off system (not shown). The noise which isproduced by the force transmitter and the tapping-off systems is in thiscase indicated schematically by the noise 1 (reference symbol 5) and thenoise 2 (reference symbol 6).

The Coriolis gyro 1 furthermore has four control loops:

A first control loop is used for controlling the stimulation oscillation(that is to say the frequency of the first resonator 3) at a fixedfrequency (resonant frequency). The first control loop has a firstdemodulator 7, a first low-pass filter 8, a frequency regulator 9, a VCO(voltage controlled oscillator) 10 and a first modulator 11.

A second control loop is used for controlling the stimulationoscillation at a constant amplitude and has a second demodulator 12, asecond low-pass filter 13 and an amplitude regulator 14.

A third and a fourth control loop are used for resetting those forceswhich stimulate the read oscillation. In this case, the third controlloop has a third demodulator 15, a third low-pass filter 16, aquadrature regulator 17 and a second modulator 18. The fourth controlloop contains a fourth demodulator 19, a fourth low-pass filter 20, arotation rate regulator 21 and a third modulator 22.

The first resonator 3 is stimulated at its resonant frequency 1. Theresultant stimulation oscillation is tapped off, is demodulated in phaseby means of the first demodulator 7, and a demodulated signal componentis passed to the first low-pass filter 8, which removes the sumfrequencies from it. The tapped-off signal is also referred to in thefollowing text as the tapped-off stimulation oscillation signal. Anoutput signal from the first low-pass filter 8 is applied to a frequencyregulator 9, which controls the VCO 10 as a function of the signal thatis supplied to it such that the in-phase component essentially tends tozero. For this purpose, the VCO 10 passes a signal to the firstmodulator 11, which itself controls a force transmitter such that thefirst resonator 3 has a stimulation force applied to it. If the in-phasecomponent is zero, then the first resonator 3 oscillates at its resonantfrequency 1. It should be mentioned that all of the modulators anddemodulators are operated on the basis of this resonant frequency 1.

The tapped-off stimulation oscillation signal is, furthermore, passed tothe second control loop and is demodulated by the second demodulator 12,whose output is passed through the second low-pass filter 13, whoseoutput signal is in turn supplied to the amplitude regulator 14. Theamplitude regulator 14 controls the first modulator 11 as a function ofthis signal and of a nominal amplitude transmitter 23 such that thefirst resonator 3 oscillates at a constant amplitude (that is to say thestimulation oscillation has a constant amplitude).

As has already been mentioned, movement/rotation of the Coriolis gyro 1results in Coriolis forces—indicated by the term FCcos(1·t) in thedrawing—which couple the first resonator 3 to the second resonator 4,and thus cause the second resonator 4 to oscillate. A resultant readoscillation at the frequency 2 is tapped off, so that a correspondingtapped-off read oscillation signal (read signal) is supplied both to thethird control loop and to the fourth control loop. In the third controlloop, this signal is demodulated by means of the third demodulator 15,the sum frequencies are removed by the third low-pass filter 16, and thelow-pass-filtered signal is supplied to the quadrature regulator 17,whose output signal is applied to the third modulator 22 such thatcorresponding quadrature components of the read oscillation are reset.Analogously to this, the tapped-off read oscillation signal isdemodulated in the fourth control loop by means of the fourthdemodulator 19, passes through the fourth low-pass filter 20, and acorrespondingly low-pass-filtered signal is applied on the one hand tothe rotation rate regulator 21, whose output signal is proportional tothe instantaneous rotation rate, and which is passed as the rotationrate measurement result to a rotation rate output 24, and is applied onthe other hand to the second modulator 18, which resets correspondingrotation rate components of the read oscillation.

A Coriolis gyro 1 as described above may be operated not only in adouble-resonant form but also in a form in which it is notdouble-resonant. If the Coriolis gyro 1 is operated in a double-resonantform, then the frequency 2 of the read oscillation is approximatelyequal to the frequency 1 of the stimulation oscillation while, incontrast, when it is operated in a form in which it is notdouble-resonant, the frequency 2 of the read oscillation differs fromthe frequency 1 of the stimulation oscillation. In the case ofdouble-resonance, the output signal from the fourth low-pass filter 20contains corresponding information about the rotation rate, while, whenit is not operated in a double-resonant form, on the other hand, it isthe output signal from the third low-pass filter 16. In order to switchbetween the different double-resonant/not double-resonant modes, adoubling switch 25 is provided, which connects the outputs of the thirdand fourth low-pass filters 16, 20 selectively to the rotation rateregulator 21 and to the quadrature regulator 17.

When the Coriolis gyro 1 is intended to be operated in a double-resonantform, the frequency of the read oscillation must be tuned—asmentioned—to the frequency of the stimulation oscillation. This may beachieved, for example, by mechanical means, in which material is removedfrom the mass system (to the resonator 2). As an alternative to this,the frequency of the read oscillation can also be set by means of anelectrical field, in which the resonator 2 is mounted such that it canoscillate, that is to say by changing the electrical field strength. Itis thus possible to electronically tune the frequency of the readoscillation to the frequency of the stimulation oscillation duringoperation of the Coriolis gyro 1, as well.

The object on which the invention is based is to provide a method bymeans of which the frequency of the read oscillation in a Coriolis gyrocan be electronically tuned to the frequency of the stimulationoscillation.

This object is achieved by the method as claimed in the features ofpatent claim 1. The invention furthermore provides a Coriolis gyro asclaimed in patent claim 10. Advantageous refinements and developments ofthe idea of the invention can be found in the respective dependentclaims.

According to the invention, in the case of a method for electronictuning of the frequency of the read oscillation to the frequency of thestimulation oscillation in a Coriolis gyro, the resonator of theCoriolis gyro has a disturbance force applied to it such that a) thestimulation oscillation remains essentially uninfluenced, and b) theread oscillation is changed such that a read signal which represents theread oscillation contains a corresponding disturbance component, whereinthe frequency of the read oscillation is controlled such that any phaseshift between a disturbance signal which produces the disturbance forceand the disturbance component which is contained in the read signal isas small as possible.

In this case, the wording “resonator” means the entire mass system (or apart of it) which can be caused to oscillate in the Coriolis gyro—thatis to say that part of the Coriolis gyro which is annotated with thereference number 2.

A significant discovery on which the invention is based is that the“time for disturbance to pass through”, that is to say an artificialchange to the read oscillation resulting from the application ofappropriate disturbance forces to the resonator, the resonator, that isto say the time which passes from the effect of the disturbance on theresonator until the disturbance is tapped off as part of the readsignal, is dependent on the frequency of the read oscillation. The shiftbetween the phase of the disturbance signal and the phase of thedisturbance component signal which is contained in the read signal isthus a measure of the frequency of the read oscillation. It can be shownthat the phase shift assumes a minimum when the frequency of the readoscillation essentially matches the frequency of the stimulationoscillation. If the frequency of the read oscillation is thus controlledsuch that the phase shift assumes a minimum, then the frequency of theread oscillation is thus at the same time essentially matched to thefrequency of the stimulation oscillation.

The significant factor in this case is that the disturbance forces onthe resonator change only the read oscillation, but not the stimulationoscillation. With reference to FIG. 2, this means that the disturbanceforces act only on the second resonator 4, but not on the firstresonator 3.

The disturbance force is preferably produced by a disturbance signalwhich is supplied to appropriate force transmitters, or is added tosignals which are supplied to the force transmitters. By way of example,a disturbance signal can be added to the respective control/resetsignals for control/compensation of the read oscillation, in order toproduce the disturbance force.

The disturbance signal is preferably an alternating signal, for examplea superimposition of sine-wave signals and cosine-wave signals. Thisdisturbance signal is generally at a fixed disturbance frequency, sothat the disturbance component of the tapped-off read oscillation signalcan be determined by means of an appropriate demodulation process, whichis carried out at the said disturbance frequency.

The method described above can be used both for an open loop and for aclosed loop Coriolis gyro. In the latter case, the disturbance signal ispreferably added to the respective control/reset signals forcontrol/compensation of the read oscillation. By way of example, thedisturbance signal can be added to the output signal from the quadraturecontrol loop, and the disturbance component can be determined from asignal which is applied to a quadrature regulator in the quadraturecontrol loop, or is emitted from it. Furthermore, it is possible to addthe disturbance signal to the output signal from the rotation ratecontrol loop, and to determine the disturbance component from a signalwhich is applied to a rotation rate regulator in the rotation ratecontrol loop, or is emitted from it. The expression “read signal” coversall signals which are described in this paragraph and from which thedisturbance component can be determined. It can also mean the tapped-offread oscillation signal.

The frequency of the read oscillation, that is to say the forcetransmission of the control forces which are required for frequencycontrol, is in this case controlled by controlling the intensity of anelectrical field in which a part of the resonator oscillates, with anelectrical attraction force between the resonator and an opposing piece,which is fixed to the frame and surrounds the resonator, preferablybeing non-linear.

The invention furthermore provides a Coriolis gyro which has a rotationrate control loop and a quadrature control loop and is characterized bya device for electronic tuning of the frequency of the read oscillationto the frequency of the stimulation oscillation. The device forelectronic tuning in this case has:

-   -   a disturbance unit which passes a disturbance signal to the        rotation rate control loop or to the quadrature control loop,    -   a disturbance signal detection unit, which determines a        disturbance component which is contained in a read signal (which        represents the read oscillation) and has been produced by the        disturbance signal, and    -   a control unit, which controls the frequency of the read        oscillation such that any phase shift between the disturbance        signal and the disturbance component which is contained in the        read signal is as small as possible.

The disturbance unit preferably passes the disturbance signal to therotation rate control loop, and the disturbance signal detection unitdetermines the disturbance component from a signal which is applied to arotation rate regulator in the rotation rate control loop, or is emittedfrom it. A further alternative is for the disturbance signal to bepassed by the disturbance unit to the quadrature control loop, with thedisturbance signal detection unit then determining the disturbancecomponent from a signal which is applied to a quadrature regulator inthe quadrature control loop, or is emitted from it.

One exemplary embodiment of the invention will be explained in moredetail in the following text with reference to the accompanying figures,in which:

FIG. 1 shows the schematic design of a Coriolis gyro which is based onthe method according to the invention; and

FIG. 2 shows the schematic design of a conventional Coriolis gyro.

First of all, one exemplary embodiment of the method according to theinvention will be explained in more detail with reference to FIG. 1. Inthis case, parts and/or devices which correspond to those in FIG. 2 areidentified by the same reference symbols, and will not be explained onceagain.

A Coriolis gyro 1′ is additionally provided with a disturbance unit 26,a first demodulation unit 27, a read oscillation frequency regulator 28,a read oscillation modulation unit 29, a second demodulation unit 30 anda modulation correction unit 31.

The disturbance unit 26 produces a first disturbance signal, preferablyan alternating signal at a frequency mod, which is added to the outputsignal from a rotation rate regulator 21 (that is to say at the forceoutput from the rotation rate control loop). The collated signal whichis obtained in this way is supplied to a modulator 18 (secondmodulator), whose corresponding output signal is applied to theresonator 2 by means of a force transmitter (not shown). The alternatingsignal is additionally supplied to the first demodulation unit 27.

The tapped-off read oscillation signal is demodulated by a fourthdemodulator 19, the output signal from the fourth demodulator beingapplied to a fourth low-pass filter 20, whose output signal is suppliedto a rotation rate regulator 21. An output signal from the rotation rateregulator 21 is supplied both to the second modulator 18 and to thefirst demodulation unit 27, which demodulates this signal based on themodulation frequency mod which corresponds to the frequency of thealternating signal which is produced by the disturbance unit 26 and thedisturbance component or the alternating signal which represents thedisturbance produced by the disturbance unit 26 is thus determined. Inparticular, the first demodulation unit 27 determines the phase of thedisturbance component signal contained in the read signal, and comparesthis with the phase of the disturbance signal which is produced by thedisturbance unit 26. The phase shift calculated in this way is suppliedto the read oscillation frequency regulator 28, which adjusts thefrequency of the read oscillation such that the phase shift is aminimum. In order to regulate the phase shift at a minimum, theelectronically tunable frequency of the read oscillation is modulatedwith a second disturbance signal ω2-Mod by the read oscillationmodulation unit 29. This results in the phase shift being varied inaccordance with this second disturbance signal. The phase shift from thefirst demodulation unit 27 is now demodulated corresponding to thesecond disturbance signal ω2-Mod. If the phase shift from the firstdemodulation unit 27 is substantially a minimum, then the signal at theinput of the read oscillation frequency regulator 28 is essentiallyzero. If, in contrast, the phase shift is not a minimum, then thisresults in a signal other than zero at the input of the read oscillationfrequency regulator 28 and with a corresponding mathematical sign, sothat the read oscillation frequency regulator 28 minimizes the phaseshift by means of the electronic frequency control. When a minimum suchas this has been reached, then the frequencies of the stimulationoscillation and of the read oscillation essentially match.

As already mentioned, and as an alternative to this, the alternatingsignal which is produced by the disturbance unit 26 can also be added toan output signal from the quadrature regulator 17. In this case, thesignal which is supplied to the first demodulation unit 27 would betapped off at the input or output of the quadrature regulator 17.

Furthermore, in principle, it is possible to feed the disturbance signalinto the quadrature control loop/rotation rate control loop at anydesired point (not only directly upstream of the second or thirdmodulator 18, 22), that is to say at any desired point between the pointat which the read oscillation is tapped off and the second or thirdmodulator 18, 22.

Once the Coriolis gyro 1′ has been switched on, it is advantageous toset the modulation frequency mod of the alternating signal to a highvalue in order to quickly achieve coarse control of the read oscillationfrequency. It is then possible to switch to a relatively low modulationfrequency mod, in order to precisely set resonance of the readoscillation. Furthermore, the amplitude of the modulation frequency modcan be greatly reduced a certain time after stabilization of therotation rate regulator 21 and/or of the quadrature regulator 17.

In principle, all the modulation processes can also be carried out onthe basis of band-limited noise. This means that all the alternatingsignals described above (the first disturbance signal ωmod and thesecond disturbance signal ω2-Mod) can be replaced by corresponding noisesignals, with the corresponding demodulation processes in this casebeing carried out on the basis of cross-correlation, that is to say onthe basis of a correlation between the noise signals and the readsignal, which contains noise components (disturbance components)produced by the noise signals.

In the case of a second alternative method for electronic tuning of thefrequency of the read oscillation to the frequency of the stimulationoscillation in a Coriolis gyro, a disturbance force is applied to theresonator of the Coriolis gyro in such a way that a) the stimulationoscillation remains essentially uninfluenced, and b) the readoscillation is changed such that a read signal which represents the readoscillation contains a corresponding disturbance component, wherein thefrequency of the read oscillation is controlled such that the magnitudeof the disturbance component which is contained in the read signal is assmall as possible.

A significant discovery on which the invention is based is that anartificial change to the read oscillation in the rotation rate channelor quadrature channel is visible to a greater extent, in particular inthe respective channel which is orthogonal to this, the less the extentto which the frequency of the read oscillation matches the frequency ofthe stimulation oscillation. The “penetration strength” of a disturbancesuch as this to the tapped-off read oscillation signal (in particular tothe orthogonal channel) is thus a measure of how accurately thefrequency of the read oscillation is matched to the frequency of thestimulation oscillation. Thus, if the frequency of the read oscillationis controlled such that the penetration strength assumes a minimum, thatis to say such that the magnitude of the disturbance component which iscontained in the tapped-off read oscillation signal is a minimum, thenthe frequency of the read oscillation is thus at the same timeessentially matched to the frequency of the stimulation oscillation.

The significant factor in this case is that the disturbance forces onthe resonator change only the read oscillation, but not the stimulationoscillation. With reference to FIG. 2, this means that the disturbanceforces act only on the second resonator 4, but not on the firstresonator 3.

In a third alternative method for electronic tuning of the frequency ofthe read oscillation to the frequency of the stimulation oscillation ina Coriolis gyro, the resonator of the Coriolis gyro has a disturbanceforce applied to it such that a) the stimulation oscillation remainsessentially uninfluenced and b) the read oscillation is changed suchthat a read signal which represents the read oscillation contains acorresponding disturbance component, with the disturbance force beingdefined as that force which is caused by the signal noise in the readsignal. The frequency of the read oscillation is in this case controlledsuch that the magnitude of the disturbance component which is containedin the read signal, that is to say the noise component, is as small aspossible.

The word “resonator” in this case means the entire mass system which canbe caused to oscillate in the Coriolis gyro—that is to say that part ofthe Coriolis gyro which is identified by the reference number 2. Theessential feature in this case is that the disturbance forces on theresonator change only the read oscillation, but not the stimulationoscillation. With reference to FIG. 2, this would mean that thedisturbance forces acted only on the second resonator 4, but not on thefirst resonator 3.

A significant discovery on which the third alternative method is basedis that a disturbance signal in the form of signal noise, which occursdirectly in the tapped-off read oscillation signal or at the input ofthe control loops (rotation rate control loop/quadrature control loop)can be observed to a greater extent in the tapped-off read oscillationsignal after “passing through” the control loops and the resonator, theless the extent to which the frequency of the read oscillation matchesthe frequency of the stimulation oscillation. The signal noise, which isthe signal noise of the read oscillation tapping-off electronics or therandom walk of the Coriolis gyro, is applied, after “passing through”the control loops, to the force transmitters and thus producescorresponding disturbance forces, which are applied to the resonator andthus cause an artificial change in the read oscillation. The“penetration strength” of a disturbance such as this to the tapped-offread oscillation signal is thus a measure of how accurately thefrequency of the read oscillation is matched to the frequency of thestimulation oscillation. Thus, if the frequency of the read oscillationis controlled such that the penetration strength assumes a minimum, thatis to say the magnitude of the disturbance component which is containedin the tapped-off read oscillation signal, that is to say the noisecomponent, is a minimum, then the frequency of the read oscillation isat the same time thus matched to the frequency of the stimulationoscillation.

The first method according to the invention which was described forelectronic tuning of the read oscillation frequency can be combined asrequired with the second alternative method and/or with the thirdalternative method. For example, it is possible to use the methoddescribed first while the Coriolis gyro is being started up (rapidtransient response), and then to use the third alternative method (slowcontrol process) in steady-state operation. Specific technicalrefinements as well as further details relating to the methods can befound by those skilled in the art in the patent applications “Verfahrenzur elektronischen Abstimmung der Ausleseschwingungsfrequenz einesCorioliskreisels”, [Method for electronic tuning of the read oscillationfrequency of a Coriolis gyro], LTF-190-DE and LTF-192-DE from the sameapplicant, in which, respectively, the second alternative method and thethird alternative method are described. The entire contents of thepatent applications LTF-190-DE/LTF-192-DE are thus hereby included inthe description.

1. A method for electronic tuning of the frequency of the readoscillation to the frequency of the stimulation oscillation in acoriolis gyro wherein the resonator of the coriolis gyro has adisturbance force applied to it such that a) the stimulation oscillationremains essentially uninfluenced, and b) the read oscillation is changedsuch that a signal which represents the read oscillation contains acorresponding disturbance component, wherein the frequency of the readoscillation is controlled such that any phase shift between adisturbance signal which produces the disturbance force and thedisturbance component which is contained in the read signal is as smallas possible.
 2. The method as claimed in claim 1, characterized in thatthe disturbance force is produced by a disturbance signal which is addedto the respective control/reset signals for control/compensation of theread oscillation.
 3. The method as claimed in claim 1, characterized inthat the disturbance signal is an alternating signal.
 4. The method asclaimed in claim 3, characterized in that the disturbance signal is at afixed disturbance frequency, and the disturbance component is determinedfrom the read signal by demodulation of the read signal at the fixeddisturbance frequency.
 5. The method as claimed in claim 1,characterized in that the disturbance signal is a band-limited noisesignal.
 6. The method as claimed in claim 5, characterized in that thedisturbance component is demodulated from the read signal by correlationof the disturbance signal with the read signal.
 7. The method as claimedin claim 2, characterized in that the disturbance signal is added to theoutput signal from the quadrature control loop, and the disturbancecomponent is determined from a signal which is applied to a quadratureregulator in athe quadrature control loop, or is emitted from it.
 8. Themethod as claimed in claim 2, characterized in that the disturbancesignal is added to the output signal from the rotation rate controlloop, and the disturbance component is determined from a signal which isapplied to a rotation rate regulator in the rotation rate control loop,or is emitted from it.
 9. The method as claimed in claim 2,characterized in that the frequency of the read oscillation iscontrolled by controlling the intensity of an electrical field in whicha part of the resonator of the Coriolis gyro oscillates.
 10. A Coriolisgyro which has a rotation rate control loop and a quadrature controlloop, characterized by a device for electronic tuning of the frequencyof the read oscillation to the frequency of the stimulation oscillation,having: a disturbance unit which passes a disturbance signal to therotation rate control loop or to the quadrature control loop, adisturbance signal detection unit, which determines a disturbancecomponent which is contained in a read signal (which represents the readoscillation) and has been produced by the disturbance signal, and acontrol unit, which controls the frequency of the read oscillation suchthat any phase shift between the disturbance signal and the disturbancecomponent which is contained in the read signal is as small as possible.11. The Coriolis gyro as claimed in claim 10, characterized in that thedisturbance unit passes the disturbance signal to the rotation ratecontrol loop, and the disturbance signal detection unit determines thedisturbance component from a signal which is applied to a rotation rateregulator in the rotation rate control loop, or is emitted from it. 12.The Coriolis gyro as claimed in claim 10, characterized in that thedisturbance unit passes the disturbance signal to the quadrature controlloop, and the disturbance signal detection component from a signal whichis applied to a quadrature regulator in the quadrature control loop, oris emitted from it.